gap between diastolic and systolic

I'm doing a research on blood pressure using journals.To be honest I'm a bit confused because some of the articles are saying that the bigger the gap between the systolic and diastolic is the better it is. I don't really understand why. Those article are saying that the lower the blood pressure is the longer you will live. So what is the limit for BP to be healthy?
Thank you

For me, I want my blood pressure to average around 120/70 24 hours around the clock, but at the same time, I don't want that low level of blood pressure to be the result of poor health and a poor circulatory system.

If the upper number, the Systolic is at an unhealthy level, it means that the body is having problems pumping the blood through the circulatory system. If the lower number, the Diastolic, is at an unhealthy average, it means that the body is under unhealthy pressure almost all the time.